Studies were undertaken to determine the prevalence and natural behavior of type C RNA tumor viruses of chicken, mouse, cat and man. Inoculation of avian sarcoma virus transformed but "virus-free" hamster tumor cells into chicks resulted in the rescue of viral gene sequences resulting in expression of avian sarcoma virus in the chicken cells. In vitro transformation of murine B-lymphocytes by Abelson virus was studied and a series of antigen-binding splenic B-lymphocytic cell lines were obtained. Hybrid cell lines were developed to map the integration site of Abelson virus in mouse chromosomes. Similar studies with somatic cell hybrids were carried out to map the integration site of woolly monkey sarcoma virus and feline leukemia virus in human chromosomes. Caffeine was shown to inhibit RNA-dependent DNA polymerase and its effect on in vitro transformation of mammalian cells by FeSV is currently being investigated.